Hypersonic Composites Resist Extreme Heat and Stress
- Tuesday, 01 January 2008
On October 14, 1947, Captain Charles “Chuck” Yeager made history when he became the first pilot in an officially documented flight to ever break the sound barrier. Flying a Bell XS-1 test jet over the Mohave Desert, Yeager hit approximately 700 miles per hour, when a loud boom thundered across the barren landscape as he crossed from subsonic to supersonic speeds. The sonic boom, akin to the wake of the plane’s shockwaves in the air, occurred at Mach 1—the speed of sound (named after Ernst Mach, an Austrian physicist whose work focused on the Doppler effect and acoustics).
This eventually led to the famous 3-hour trans-Atlantic flights of the Concorde, traveling at Mach 2, and the development of fighter jets, which began routinely crossing the sound barrier—the era of supersonic flight. At NASA, however, aerospace engineers wanted to go faster—hypersonic or Mach 5; five times the speed of sound. They continued to push the limits of speed, setting a world record in October 1967, which held for 35 years at Mach 6.7, in the NASA-designed X-15 aircraft.
The major experimental feature of this particular aircraft was a scramjet engine, which kicks in at about Mach 6. In a normal jet engine, blades compress the air, but in the scramjet, the combustion of hydrogen fuel in a stream of air is compressed by the high speed of the aircraft itself. To achieve Mach 10, the X-43A was first carried up into the air attached to a B-52 and then “jumpstarted” with a Pegasus rocket.
It was a multiyear experiment and involved building three test vehicles. The first two were meant to reach speeds of up to Mach 7, and the third was reaching for double digits: Mach 10. NASA hoped that these three unmanned crafts would allow aerospace engineers to further advance understanding of hypersonic flight and that the lessons learned could be applied to increase payload capacity for future vehicles, including hypersonic aircraft and reusable space launchers.
Tullahoma, Tennessee-based ATK-GASL built the X-43A and its engine, while the Huntington Beach, California-based Boeing Company’s Phantom Works designed the thermal protection and onboard system. The booster, a modified Pegasus rocket, was built by Orbital Sciences Corporation at the company’s facility in Chandler, Arizona. The program was jointly managed by NASA’s Dryden Flight Research Center and Langley Research Center.
MER created all of the leading edges for the X-43A test vehicles at Dryden. Considered the most critical parts of this experimental craft, the leading edges had several specific requirements. As the vehicle’s speed increased, so did heat and thermal load, approaching 4,000 °F, well above the temperatures the shuttle is exposed to during reentry. In addition to being very heat resistant, the coating had to be very lightweight and thin, as the aircraft was designed to very precise specifications and could not afford to have a bulky coating.
In total, 11 C-C leading edges were created for the nose, the chines (the areas where the body meets the wing), the verticals, and 2 horizontals. These parts were made with P-30X graphite fibers, using a liquid matrix process. A very high process temperature was utilized to render the high-thermal conductivity. The parts were first processed in the form of a C-C billet, and then machined to NASA specifications. Oxidation protection was achieved by a dual chemical vapor reaction and chemical vapor deposition process.
To take advantage of commercializing these specialized composites, MER patented its C-C composite process and then formed a spinoff company, Frontier Materials Corporation (FMC), in Tucson, Arizona. FMC is using the patent in conjunction with low-cost PAN (polyacrylonitrile)-based fibers to introduce these materials to the commercial markets.
The C-C composites are very lightweight, yet still have great strength and stiffness, even at very high temperatures. They can be produced with either low- or high-thermal conductivity, and when graphitized, they have superior electrical conductivity. Even with all of these characteristics, the carbon composites are still relatively inexpensive.
The composites have been used in industrial heating applications, the automotive and aerospace industries, as well as in glass manufacturing and on semiconductors. C-C composites have been used in industrial heating fixtures in the form of structural flat panels, structural members for panel support, and for nuts and bolts. In the automotive industry, the composites have been useful for engine components, brakes, cylinder liners, and panels. Aerospace applications have included missile bodies, leading edges, and structured components. Applications also include transfer components for glass manufacturing and structural members for carrier support in semiconductor processing.